Blank TextField and LongTextField

[piotrpospiech]

Closes #1332

• Deleting input from TextField and LongTextField sets undefined instead of empty string in the model
• This behavior is already implemented in other fields (playground with optional TextField and NumField)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.03%. Comparing base (e1c4707) to head (09e150d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1356      +/-   ##
==========================================
+ Coverage   95.02%   95.03%   +0.01%     
==========================================
  Files         175      175              
  Lines        2892     2900       +8     
  Branches      763      777      +14     
==========================================
+ Hits         2748     2756       +8     
  Misses         59       59              
  Partials       85       85              

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